1. Field of the Invention
The invention relates to a method of improving the surface insulation resistance of an electrical steel having an insulative surface coating thereon, and more particularly to the subjecting of an electrical steel to at least one electrochemical treating step to remove small metallic particles, nodules or the like extending through or protruding above the insulative coating and which can result in increased watt loss in laminated magnetic structures excited with alternating current because of reduced resistance to interlaminar current flow.
2. Description of the Prior Art
The present invention is applicable to oriented silicon steels with a mill glass coating, carbon steels for electrical uses having an insulative coating thereon, and cold rolled non-oriented silicon steels with an applied insulative coating. The terms "electrical steel" or "electrical steels," as used herein and in the claims, is to be interpreted as encompassing the above noted types of steels. For purposes of an exemplary showing, the present invention will be described in its application to the manufacture of oriented silicon steels. As used herein and in the claims, the term "oriented silicon steel" refers to silicon steel wherein the body-centered cubes making up the grains or crystals are oriented in a cube-on-edge position, designated (110) [001] in accordance with Miller's indices.
Oriented silicon steels are well known in the art and have been chosen for purposes of an exemplary teaching of the present invention because in their typical applications, as for exmaple in the manufacture of transformer cores and the like, surface insulation resistance is of considerable importance.
In recent years prior art workers have devised various routings for the manufacture of oriented silicon steel which have resulted in markedly improved magnetic characteristics. As a result, such oriented silicon steels are now considered to fall into two general catagories. The first catagory is usually referred to as high permeability oriented silicon steel and is made by routings which consistently produce a product having a permeability at 796A/m of greater that about 1850 and typically greater than about 1900. U.S. Pat. No. 3,287,183; 3,636,579; 3,873,234 are typical of those which teach routings for high permeability oriented silicon steel. The second catagory is generally referred to as regular oriented silicon steel and is made by those routings normally producing a permeability of less than about 1850. U.S. Pat. No. 3,764,406 is typical of those which set forth routings for regular oriented silicon steel. The teachings of the present invention are applicable to both types of oriented silicon steel.
With both types of oriented silicon steel the basic steps of the manufacturing process or routing include casting a melt into ingots which are rolled into slabs or continuously casting the melt into slab form. The slabs are reheated, hot rolled to hot band thickness, annealed and cold rolled to final gauge in one or more stages. Following cold rolling, the silicon steel is subjected to a decarburizing step, provided with an annealing separator and subjected to a final box anneal during which the desired final magnetic characteristics are for the most part achieved.
While the above lists the basic steps of the routings for oriented silicon steel, other steps may be included and the precise nature of the routing does not constitute a limitation on the present invention.
In the manufacture of high permeability oriented silicon steel an exemplary melt composition in weight percent may be stated as follows:
Si 2%-4% PA1 C less than 0.085% PA1 Al (Acid-soluble) up to 0.065% PA1 N 0.003%-0.010% PA1 mn 0.02%-0.2% PA1 S and/or Se 0.015%-0.07% PA1 B up to 0.012% PA1 Cu up to 0.5% PA1 C less than 0.085% PA1 Si 2%-4% PA1 S and/or Se 0.015%-0.07% PA1 Mn 0.02%-0.2%
Similarly, in the manufacture of regular oriented silicon steel, a typical melt composition by weight percent may be set forth as follows:
In the manufacture of either type of oriented silicon steel the most common practice is to provide, prior to the final anneal, an annealing separator which (during the final anneal) will form an insulative glass film on the surfaces of the oriented silicon steel. Magnesia, for example, is a typical annealing separator which forms an insulative glass film, as taught in U.S. Pat. Nos. 2,385,332 and 2,906,645. Other exemplary annealing separators are set forth in U.S. Pat. Nos. 3,544,396 and 3,615,918. The insulative glass coating formed by such annealing separators is generally known in the art as a "mill glass". For purposes of this description, such insulative coatings will be termed "primary coatings".
In the manufacture of carbon steels for electrical applications and cold rolled non-oriented silicon steels, a surface insulative coating may be applied. This coating may be of the type caught in U.S. Pat. Nos. 2,501,846 and 3,996,073, or an organic type as taught in U.S. Pat. Nos. 3,865,616; 3,853,971 and 3,908,066. These coatings, which are applied to improve the interlaminar resistance, are intended to be included in the term "primary coatings," as used herein and in the claims.
Excellent surface insulation resistance, or low amperes by the ASTM test method A717 (commonly referred to as the Franklin resistivity test method) is impaired by the presence of small metallic particles or the like extending through or protruding above the surface of the primary insulative coating. The present invention is based upon the discovery that if the oriented silicon steel, having a mill glass formed thereon, is subjected to a continuous electrochemical treatment step, an improvement in surface insulation will occur by virtue of the fact that any small metallic particles extending through or protruding above the mill glass are removed without harming the insulative characteristics of the primary insulative coating or mill glass. Depending upon the quality of the primary insulative coating, average surface insulation resistance improvements equivalent to a change in current of from about 0.67 to about 0.34 amps by ASTM test method 717 may be achieved.
In addition, it is usual practice in the manufacture of transformer cores and the like to provide a secondary coating over the primary coating. Exemplary secondary coatings are taught in U.S. Pat. Nos. 2,501,846 and 3,996,073. A primary function of such applied secondary coatings is to reduce interlaminar eddy currents. With the practice of the present invention less secondary coating may be required since there will be no metallic particles or the like extending through or protruding above the surface of the primary insulative coating. This results not only in a savings of material, but also in the improvement of the space factor characteristics of the oriented silicon steel. A heavy secondary coating is to be avoided since it results in increased cost, a tendency to powder, drying problems, furnace maintenance problems and pimpling of the secondary coating.